The present invention relates to a heater fabric and method of making the same.
For many years the only satisfactory source of a semi-flexible heater material was a product known as a heater cable. By arranging these heater cables in equally spaced configurations it was possible to provide a large flat area of radiant heat source. These heater cables were all made by insulating an alloy resistance wire, such as Chromel (Trademark of Haskins Mfg. Co. for a series of Ni-Cr alloys) and then encasing the insulated heater wire in a metallic sheath.
More recently, there have been many attempts to develop other types of flexible heater products, such as woven graphite fibers, sheets of conducting material sandwiched between a film or fabric coated with conducting compounds and enclosing in insulating film. See U.S. Pat. Nos. 3,146,340 and 3,400,254.
In all cases, when these products were put in service it was necessary to encase them in protective film to protect them from moisture and from the hazardous contact with the surface of the "live" electric circuit. However, the presence of this film is a serious deterent to its use in many applications, such as molded ceiling tile, plaster board, etc.
U.S. Pat. No. 2,473,183 teaches coating a woven fabric with a vinyl resin containing conducting carbon black, plus an over coating of a non-conductive material, such as plasticized vinyl resin to protect a person from getting electrical shock when touching the surface.
The insulating coating of this patent being a plasticized vinyl resin will not sufficiently protect the conducting coating since a 3 mil vinyl coating would have a moisture vapor transmission of from 0.5 to 1.0 perms.
U.S. Pat. No. 3,112,985 teaches the bonding of a film of polyvinylidene chloride to paper for the purpose of making it greaseproof and resistant to the passage therethrough of water vapor.
U.S. Pat. No. 3,359,525 teaches coating glass fabric with a conductive coating of polyimide resin and conducting carbon black followed by an insulating coating of a polyimide resin. This provides an insulating coating over the conductive coating, but not a good vapor varrier. This product is designed to operate at 600.degree.F.
None of these products has proven satisfactory, as witnessed by the fact that the Underwriters' Laboratories, Inc. do not include any such heater devices on its list of approved radiant heat sources.
The heater cable is still the only recognized and approved source of radiant heat for installation in homes, driveways, etc.
It is the object of this invention to produce a flexible heat source that will satisfactorily overcome all the previous objections and provide a reliable heat source in the range of 10 to 70 watts per square foot. Such a product would be suitable for a large variety of end uses, such as home heating in the form of a ceiling tile or ceiling board, preventing ice on bridges, roadways, and airport runways, heated plant beds, matress pads, heated storage tanks, etc.